Charged species in solution or suspension in a liquid will migrate under the influence of an applied electric field in a phenomenon known as electrophoresis. Different species have different electrophoretic mobilities, providing a mechanism for separating different species. Coupling this separation means with a detecting means gives an analytical technique known as capillary zone electrophoresis.
Electroosmosis is the flow of liquid, generally a polar liquid, in contact with a porous solid, under the influence of an applied electric field. Electroosmosis has been attributed to the formation of an electric double layer at the solid/liquid interface. Under the influence of an electric field parallel to that interface, a portion of the liquid's diffuse layer moves, because of the electric forces acting on the excess ionic charge in that layer, and a shear plane is set up at some distance from the interface. A constant flow rate is reached when the force exerted on the ions (and thus on the liquid as a whole) is balanced by the frictional forces arising from the viscosity of the liquid. The electric potential at the shear plane is called the zeta potential, represented by z or the Greek letter "zeta."
The linear velocity, v, of the liquid under the influence of an applied electric field E is approximately EQU v=(e/h)E z (1)
where e is the dielectric constant of the liquid, and h is the viscosity of the liquid.
Thus v is proportional (or approximately proportional) to the zeta potential, z. The magnitude of the zeta potential depends on, among other things, the particular liquid, the nature of the solid surface, and the concentrations of different species in the liquid. Polar solvents, such as water, can give rise to zeta potentials of as much 100 mV in contact with either polar or non-polar surfaces.
Electroosmosis can either augment or interfere with capillary electrophoresis. In can interfere because electroosmotic mobility is typically greater than are electrophoretic mobilities, limiting the time available for electrophoretic processes to separate different species before electroosmosis flushes the species out of the capillary.
One method to control the rate of electroosmosis is to change the voltage applied across the length of the capillary, but such a change also affects the rate of electrophoresis, and will not significantly affect the overall degree of electrophoretic separation. Other methods of controlling the rate of electroosmosis are to change the concentrations of species in solution or suspension, to change the pH, or to change the nature of the material forming the inner layer of the capillary. None of these methods is flexible or capable or rapid change, and each has other disadvantages as well.
There have been a number of recent efforts to make chemical sensors using microfabrication techniques. Most microsensors fabricated to data have used a two-step detection means. In the first step, chemical selectivity is accomplished by means of a chemical transformation, or by physisorption onto a chemically selective coating. In the second step, a physical consequence of the first step--e.g., a release of heat, a change in optical absorption, etc.--is converted to an electrical signal by means of a suitable microtransducer. Examples of chemical microsensors are the ISFET and the CHEMFET.
In conventional chemical sensors, the analyte is measured in the presence of other species. The selectivity component of the sensor ideally should respond only to the analyte. An alternative means of chemical analysis is first to separate the analyte from background species, and then to sense it with the detector. The detector in such a case need not be selective. An example of this type of chemical sensor is the micro-gas chromatograph developed by Terry et al., "A Gas Chromatographic Air Analyzer Fabricated on a Silicon Wafer," IEEE Trans. Electron Devices, Vol. ED-26, pp. 1880-86 (1979). Such chemical sensors may be called separation-based chemical microsensors. In addition to other factors favoring miniaturization, chromatographic systems also benefit from scaling laws--generally a decrease in column diameter results in an increased separation efficiency per unit length, or faster separations with short columns.
In capillary electrophoresis, a buffer-filled capillary is placed between two buffer reservoirs, and a potential field is applied across this capillary. The electric field creates an electroosmotic flow of buffer, generally toward the cathode. The electric field also causes electrophoretic flow of ionic solutes. The electrophoretic flow will generally be at a different rate from that of the electroosmotic flow. The difference between the electrophoretic mobilities of the solutes causes their separation. The separated species may be detected when they reach the cathode or anode, typically the cathode. Ewing et al., "Capillary Electrophoresis," Analytical Chemistry, Vol. 61, No. 4, pp. 292A-303A (1989).
A disadvantage of existing methods of capillary electrophoresis is the fact that the rate of electroosmotic flow is often high enough, compared to the electrophoretic mobilities, that electroosmotic flow carries the species out of the capillary before adequate separation of the species occurs. It is known that maximum resolution results when electroosmotic mobility just balances electrophoretic mobility. It has previously been though that such an approach required a lengthy analysis time. See Ewing et al., supra, at p. 298A; Jorgenson et al., "Zone Electrophoresis in Open-Tubular Glass Capillaries," Anal. Chem. Vol. 53, pp. 1298-1302 (1981). The desirability of thus tuning electroosmosis to achieve a high-resolution separation, and then re-tuning the electroosmotic flow to sweep separated components to a detector, has been recognized in the art. See Lauer et al., "Zone Electrophoresis in Open-Tubular Capillaries--Recent Advances," Trends in Analytical Chemistry, vol. 5, no. 1, pp. 11-15 (1986), at p. 13. Despite this recognition, to the knowledge of the inventor there has been nor previously reported means for controlling the rate of electroosmosis which is simultaneously flexible, capable of rapid response, and capable of changing the rate of electroosmosis independently of the rate of electrophoresis.
Because the rate of electroosmotic flow is proportional to the zeta potential, the rate of electroosmotic flow may be controlled or even stopped by adjusting the zeta potential, preferably without changing the electric potential across the length of the capillary. But existing method of controlling the zeta potential having significant disadvantages. These methods include changing the concentrations of species in solution or suspension, to change the pH, or to change the nature of the material forming the inner layer of the capillary. None of these methods is flexible or capable or rapid change, and each has other disadvantages as well. For example, a zero zeta potential may be desired to eliminate electroosmosis, so that only electrophoresis occurs. In a silica capillary, an approximately zero zeta potential may be achieved with a pH of 2-3; but such an acidic environment denatures many proteins. See Jorgenson et al., supra, at p. 1301; McCormick, "Capillary Zone Electrophoretic Separation of Peptides and Proteins Using Low pH Buffers in Modified Silica Capillaries," Anal. Chem., Vol. 60, pp. 2322-28 (1988).
It has been observed that adding micelles to the liquid can allow the separation of neutral species as well as charged ones: A micelle may incorporate a neutral species into its interior, while the charge on the micelle's surface causes the micelle-neutral species complex to act as a charged species in the electrophoresis, permitting separations. See Terabe et al., "Electrokinetic Separations with Micellar Solutions and Open-Tubular Capillaries, " Anal. Chem., vol. 56, pp. 111-13 (1984); and Terabe et al. "Electrokinetic Chromatography with Micellar Solution and Open-Tubular Capillary," Anal. Chem., vol. 57, pp. 834-41 (1985); both of which are incorporated by reference.